An electrophotographic device (copying machine, facsimile device, printer and the like) has an image forming device and a fixing device for fixing an image formed and transferred onto a sheet by the image forming device. The fixing device includes a heat roller.
A heat roller is formed of a metallic ring member, rubber covering the metallic ring member and a halogen lamp arranged inside the metallic ring member. However, the halogen lamp is low in thermal efficiency, and moreover, the rubber covering the metallic ring member reduces the thermal efficiency. In addition, it takes several ten seconds to several minutes to reach a predetermined temperature, so that a preheating is required during a stand-by period.
Recently, there has been developed a directly-heated heat roller including a sheet-like heating element in which a resistance member is embedded in an insulating member. This heat roller has high thermal efficiency, since the resistance member generates heat when electric current flows through the resistance member and the heat is conducted. The sheet-like heating element is at first formed as a flat heating sheet. The heating sheet is rounded to form a cylindrical sheet-like heating element. The sheet-like heating element cannot keep its cylindrical shape with this state, so that it is attached on an inner surface of a metallic cylindrical tube for use. However, attaching the sheet-like heating element onto the inner surface of the cylindrical tube is difficult work.
Therefore, a method for fabricating a heat roller has been proposed wherein a cylindrical sheet-like heating element is sandwiched between an inner tube and an outer tube that constitute a duplex tube. Firstly, the inner tube is arranged at the inner surface side of the cylindrical sheet-like heating element, and then, the outer tube is arranged at the outer surface side of this heating element. Then, pressurized fluid is supplied to the inner tube to expand the inner tube and the sheet-like heating element toward the outer tube, whereby the sheet-like heating element is brought into intimate contact with the inner tube and the outer tube. In this fabrication process, it is unnecessary that the sheet-like heating element is brought into contact with the inner tube and with the outer tube, thereby providing a simple assembling operation.
There has been a demand for enhancing thermal efficiency by improving the heat roller including the sheet-like heating element.